An ink jet printer produces images on a receiver by ejecting ink droplets onto the receiver in a raster scanning fashion. The advantages of non-impact, low noise, low energy use, and low cost operation are largely responsible for the wide acceptance of ink jet printers in the marketplace.
A typical inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink onto an ink receiver. It is known to those skilled in the art that undesirable image artifacts can arise due to small differences between the individual nozzles in a printhead. These differences in the nozzles of a print head may be caused by deviations in the physical characteristics (e.g., the nozzle diameter, the channel width or length, etc.) or the electrical characteristics (e.g., thermal or mechanical activation power, etc.) of the nozzles. These variations are often introduced during print head manufacture and assembly. The differences may cause the individual nozzles to produce ink drops that are slightly different in volume from neighboring nozzles. Larger ink drops will result in darker (increased optical density) areas on the printed page, and smaller ink drops will result in lighter (decreased optical density) areas. Due to the raster scanning fashion of the printhead, these dark and light areas will form lines of darker and lighter density often referred to as “banding” or “streaking.”
There are many techniques present in the prior art that describe methods of reducing banding artifacts caused by nozzle-to-nozzle differences. For instance, in some prior art systems drop volume variability between nozzles of a printhead has been reduced by “normalizing” each jet or nozzle within a printhead. Normalization of the printhead nozzles is accomplished by modifying the electrical signals, or driving signals, that are used to activate the individual nozzles so that all of the nozzles of the printhead generate an ink drop having substantially the same drop mass.
The inkjet printing market continues to require faster and faster printing of images, and many modifications to the basic inkjet printing engine have been investigated to accommodate this requirement. One method of printing an image faster is to use a printhead that has more nozzles. This prints more image raster lines in each movement of the printhead, thereby increasing the throughput of the printer. However, manufacturing and technical challenges ultimately limit the numbers of nozzles in a printhead. Thus, in some inkjet printers designed for high throughput, multiple printheads are used together that effectively increases the number of nozzles and offer an alternative that is easier to manufacture.
While normalization of the jets in the printhead may be effective in the generation of substantially uniform drop mass across the nozzles of an individual printhead, the “normalized” drop mass produced may vary from printhead to printhead resulting in banding or streaking of a printed image. The normal variations between printheads that may be introduced, for example, during manufacture and assembly may result in printheads that generate ink drops having differing volumes. The average drop mass difference from printhead to printhead may be as high as 2-4 ng.
Techniques have been developed in the prior art to address drop volume variation issues between print heads. For example, U.S. Pat. No. 6,154,227 to Lund teaches a method of adjusting the number of micro-drops printed in response to a drop volume parameter stored in programmable memory on the print head cartridge. Also, U.S. Pat. Nos. 6,450,608 and 6,315,383 to Sarmast et al., teach methods of detecting inkjet nozzle trajectory errors and drop volume using a two-dimensional array of individual detectors. These methods, however, require the use of sophisticated sensors and ink cartridges. The calibration time, cost, and physical space constraints may weigh against the use of these and other possible complex methods. Moreover, tests have shown that even very small printhead to printhead differences in drop mass may be seen. For example, drop mass differences as small as 0.25 ng or less has been found to be visible to the human eye. Measuring drop masses of that size approaches the limits of current measurement tools.